The present invention relates generally to radiocommunication networks, more particularly to transmission power adjustment in such networks, and concerns a downlink power control method and a cellular network applying this method.
In cellular radiocommunication networks, there exists in particular a constant need to increase capacity and to minimise the injected power for a requested signal to interference ratio (SIR) at the level of the mobile terminals.
This is particularly the case in a CDMA (Code Division Multiple Access) type network, wherein to each mobile terminal is allocated a so-called Active Set composed of the base stations received with at least a minimal power level at the mobile terminal and able to establish communication of a given quality with the latter.
In this type of networks in particular, when a mobile terminal is in handover with more than one base-station (i.e. during macrodiversity mode), this mobile presently measures the overall Eb/Io (received from all base-stations in the Active Set) and derives a single TPC (Transmit Power Control) command which is forwarded to all those base-stations in the Active Set. As a result, all these base-stations will adjust their power contribution towards the mobile following the same TPC indication (resulting in an increase/decrease of transmit power), i.e. base-stations in the Active Set always react in unison to power requests of the mobile. In particular, with such a power control scheme, power ratios between active base-stations remain constant along communication.
In practice, using single TPC commands for all base-stations connected to a mobile has shown some major drawbacks.
According to the preceding existing scheme, power distribution among active links is set for good at power initialization: once a base-station has been added to the Active Set, the fraction of power directed to the mobile by this base, as compared to the contribution of bases present in the Active set, cannot change until Active Set composition is modified.
Still, even if the Active Set is subject to modifications, the new power fraction will be determined by the addition/removal of bases in the Active Set: there is no way to control exactly the fraction of power contributed by the base. Besides, power initialization for a newly added link in a non-vacant Active Set is not covered by existing solutions.
Even considering that power initialization provides an optimum power share for the newly added link, permanency of this situation is not guaranteed. In effect, pathloss and cell-load conditions are subject to frequent changes, which requires to alter power distribution within the Active Set so as to maintain optimality. On top of this, errors in the transmission of the TPC commands might unduly modify the power share of a given link.
Sending multiple TPC commands (one for each base of the Active Set) would enable to control the power of each link independently, but is merely impractical as it would significantly increase the signalling amount on the radio interface, leading to a reduction in capacity.
In any case, the mobile station is not in a position to decide how the power should be distributed among the base-stations of the Active Set.
Hence, a slow power regulation mechanism capable of interacting with fast-power control (and possibly overruling it) is strongly needed to overcome these limitations. Preferably implemented in a supervisory entity of the network, this procedure should be in charge of updating and controlling link power allocation within the Active Set, taking possibly also into account cell-specific conditions such as loading.
It is therefore a main object of the present invention to overcome the aforementioned limitations and to meet the needs expressed herebefore.
The present invention thus concerns a power control method for adjusting, in a cellular radiocommunication network, the power transmitted by base stations BS, received with at least a preset minimal power level by a given mobile terminal MT (active set of MT), towards said mobile terminal MT, consisting in adjusting individually, preferably repeatedly, the downlink power level of each of said base stations BS according to a specific power distribution criterion, while ensuring a desired communication quality level at said mobile terminal (MT).
For example, the downlink power level of each of the base stations of the active set can be adjusted, each time, in such a way that said base stations BS are contributing approximately equally in order to reach a desired communication quality level at said mobile terminal MT.
Especially in case of a CDMA cellular network, the downlink power of each base station BS of the active set is adjusted so that each base station BS has about the same contribution in terms of the received Eb/Io (Energy per chip/Interference spectral density) at the considered mobile terminal MT.
The invention can provide additionnaly that, if one BShl of the base stations BS of the active set of the considered mobile terminal MT is affected by heavy loading conditions compared to the other base stations BS of said set, the contribution of said base station BShl to the received Eb/Io at the mobile terminal MT is reduced by a quantity function of its loading difference with the other base stations BS, the reduction quantity being approximately equally distributed among these latters.
The balancing procedure of the power distribution of the base stations of the active set of a mobile terminal is preferably repeated intermittently and, for example, the power adjustment procedure can be carried out periodically and/or each time the active set of said mobile terminal MT is modified.
As can be seen, a separate procedure is established to update downlink power distribution within the Active Set. This procedure complements fast power-control and is preferably carried out in the Radio Network Controller (RNC). It is advantageously activated upon every change in the Active Set and periodically between Active Set modifications. The goal of this procedure is to ensure an efficient power allocation within the Active Set, especially during macrodiversity periods.
The present invention also concerns a cellular radiocommunication network comprising a plurality of geographically spread base stations BS able to establish communication channels with mobile terminals MT, the said base stations BS belonging to different sets based on the power levels received at the different mobile terminals MT, wherein said network also comprises a controller which supervises power allocation in all base stations of the network and controls downlink power in each of said so-called active sets, said controller determining the downlink power transmitted by each base station BS of each active set according to the method described herebefore.